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Abstract
Previously published profiles of vertical velocity (w) skewness observed in the convective atmospheric boundary layer show deficits in the upper part of the layer, relative to large eddy simulations designed to apply to highly convective cloudless planetary boundary layers. Thus, we examine w-skewness profiles from data collected in other experiments. We find that skewness profiles in the three highly convective cases with the fewest and smallest clouds agree better with the large eddy simulation results than other profiles presented here and previously; however the deficit at the top of the boundary layer—though smaller—remains.
We hypothesize that the remaining deficit for these three cases results from the presence of ∼10-km wavelength quasi two-dimensional sinusoidal structures, which have near-zero skewness. The small domain and periodic boundary conditions of a large eddy simulation may not allow such structures to develop fully. Removal of the effects of these structures by counting only flight legs nearly parallel to their axes, for two of the cases, improves agreement between the simulation and observations. We speculate that these structures result from gravity waves interacting with the boundary layer.
Abstract
Previously published profiles of vertical velocity (w) skewness observed in the convective atmospheric boundary layer show deficits in the upper part of the layer, relative to large eddy simulations designed to apply to highly convective cloudless planetary boundary layers. Thus, we examine w-skewness profiles from data collected in other experiments. We find that skewness profiles in the three highly convective cases with the fewest and smallest clouds agree better with the large eddy simulation results than other profiles presented here and previously; however the deficit at the top of the boundary layer—though smaller—remains.
We hypothesize that the remaining deficit for these three cases results from the presence of ∼10-km wavelength quasi two-dimensional sinusoidal structures, which have near-zero skewness. The small domain and periodic boundary conditions of a large eddy simulation may not allow such structures to develop fully. Removal of the effects of these structures by counting only flight legs nearly parallel to their axes, for two of the cases, improves agreement between the simulation and observations. We speculate that these structures result from gravity waves interacting with the boundary layer.
A survey of the Journal of the Atmospheric Sciences, the Journal of Applied Meteorology, and the Monthly Weather Review shows that the number of publications per year resulting from GATE (GARP Atlantic Tropical Experiment) peaked in 1980, six years after the experiment's field phase.
A survey of the Journal of the Atmospheric Sciences, the Journal of Applied Meteorology, and the Monthly Weather Review shows that the number of publications per year resulting from GATE (GARP Atlantic Tropical Experiment) peaked in 1980, six years after the experiment's field phase.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
Based on personal experience and input from colleagues, the natural history of a field program is discussed, from conception through data analysis and synthesis of results. For convenience, the life cycle of a field program is divided into three phases: the prefield phase, the field phase, and the aftermath. As described here, the prefield phase involves conceiving the idea, developing the scientific objectives, naming the program, obtaining support, and arranging the logistics. The field phase discussion highlights the decision making process, balancing input from data and numerical models, and human interactions. The data are merged, analyzed, and synthesized into knowledge mainly after the field effort.
Three major conclusions are drawn. First, it is the people most of all who make a field program successful, and cooperation and collegial consensus building are vital during all phases; good health and a sense of humor both help make this possible. Second, although numerical models are now playing a central role in all phases of a field program, not paying adequate attention to the observations can lead to problems. And finally, it cannot be overemphasized that both funding agencies and participants must recognize that it takes several years to fully exploit the datasets collected, with the corollary that high-quality datasets should be available long term.
Abstract
Based on personal experience and input from colleagues, the natural history of a field program is discussed, from conception through data analysis and synthesis of results. For convenience, the life cycle of a field program is divided into three phases: the prefield phase, the field phase, and the aftermath. As described here, the prefield phase involves conceiving the idea, developing the scientific objectives, naming the program, obtaining support, and arranging the logistics. The field phase discussion highlights the decision making process, balancing input from data and numerical models, and human interactions. The data are merged, analyzed, and synthesized into knowledge mainly after the field effort.
Three major conclusions are drawn. First, it is the people most of all who make a field program successful, and cooperation and collegial consensus building are vital during all phases; good health and a sense of humor both help make this possible. Second, although numerical models are now playing a central role in all phases of a field program, not paying adequate attention to the observations can lead to problems. And finally, it cannot be overemphasized that both funding agencies and participants must recognize that it takes several years to fully exploit the datasets collected, with the corollary that high-quality datasets should be available long term.
Abstract
The cloud-base diameters of 40 cumulus clouds traversed by aircraft on 14 days of the Cooperative Convective Precipitation Experiment (CCOPE) are shown to increase with the vertical shear of the horizontal wind through cloud base. The relationship is stronger when only the largest clouds sampled in each of the 16 populations are considered. The relationship is strongest when the cloud diameter is normalized by the maximum achievable cloud height, as estimated by the parcel equilibrium height. Assuming a cloud diameter—height ratio of around 1, this implies that larger shear enables clouds to reach a larger fraction of their maximum possible size given the thermodynamic conditions. Alternatively, larger shear may lead to clouds with larger diameter-height ratios. The correct interpretation is probably a combination of the two.
The physical mechanisms for the growth of these largest clouds seem to involve interaction among clouds and the interaction of the clouds with cloud—and boundary layer—induced tropospheric gravity waves, as discussed by Clerk et al. (1986), since these interactions are stronger with stronger vertical shear of the horizontal wind through cloud base. Once produced, the larger clouds that produce outflows have a greater chance to enlarge or to produce new clouds in situations with stronger shear, enhancing the chance of sampling larger clouds.
Abstract
The cloud-base diameters of 40 cumulus clouds traversed by aircraft on 14 days of the Cooperative Convective Precipitation Experiment (CCOPE) are shown to increase with the vertical shear of the horizontal wind through cloud base. The relationship is stronger when only the largest clouds sampled in each of the 16 populations are considered. The relationship is strongest when the cloud diameter is normalized by the maximum achievable cloud height, as estimated by the parcel equilibrium height. Assuming a cloud diameter—height ratio of around 1, this implies that larger shear enables clouds to reach a larger fraction of their maximum possible size given the thermodynamic conditions. Alternatively, larger shear may lead to clouds with larger diameter-height ratios. The correct interpretation is probably a combination of the two.
The physical mechanisms for the growth of these largest clouds seem to involve interaction among clouds and the interaction of the clouds with cloud—and boundary layer—induced tropospheric gravity waves, as discussed by Clerk et al. (1986), since these interactions are stronger with stronger vertical shear of the horizontal wind through cloud base. Once produced, the larger clouds that produce outflows have a greater chance to enlarge or to produce new clouds in situations with stronger shear, enhancing the chance of sampling larger clouds.
Abstract
The vertical transport of horizontal momentum normal to a line of cumulonimbus observed during GATE on 14 September 1974 is against the vertical momentum gradient, contrary to the predictions of mixing-length theory. Data from repeated aircraft passes normal to the line's axis at heights from 0.15 to 5.5 km are used to document the flux and determine its source. The flux is concentrated in roughly a 25 km wide “active zone” just behind the leading edge of the line, in kilometer-scale convective updrafts accelerated upward by buoyancy and toward the rear of the line by mesoscale pressure forces. The fall in mesoscale pressure from the leading edge to the rear of the active zone is mainly hydrostatic, resulting from relatively high virtual temperatures and the 60 degree tilt of the leading edge from the vertical, with the clouds at the surface well ahead of those aloft.
Evaluation of the terms in the momentum-flux generation equation confirms that the above process, reflected by the velocity-buoyancy correlation term, is responsible for generating momentum flux of the observed sign. The component of momentum flux parallel to the axis of the convective band is generated much like “down-gradient” momentum flux within the fair-weather subcloud layer.
Abstract
The vertical transport of horizontal momentum normal to a line of cumulonimbus observed during GATE on 14 September 1974 is against the vertical momentum gradient, contrary to the predictions of mixing-length theory. Data from repeated aircraft passes normal to the line's axis at heights from 0.15 to 5.5 km are used to document the flux and determine its source. The flux is concentrated in roughly a 25 km wide “active zone” just behind the leading edge of the line, in kilometer-scale convective updrafts accelerated upward by buoyancy and toward the rear of the line by mesoscale pressure forces. The fall in mesoscale pressure from the leading edge to the rear of the active zone is mainly hydrostatic, resulting from relatively high virtual temperatures and the 60 degree tilt of the leading edge from the vertical, with the clouds at the surface well ahead of those aloft.
Evaluation of the terms in the momentum-flux generation equation confirms that the above process, reflected by the velocity-buoyancy correlation term, is responsible for generating momentum flux of the observed sign. The component of momentum flux parallel to the axis of the convective band is generated much like “down-gradient” momentum flux within the fair-weather subcloud layer.
Abstract
The wind and temperature fields of the Planetary boundary layer (PBL) are investigated during periods in which horizontal roll vortices are present. Measurements from a 444 m tower and from inertially-stabilized aircraft indicate the rolls are maintained primarily by 1) production of energy from the cross-roll component of the mean PBL wind spiral (inflectional instability and 2) buoyancy. Complicating a simple picture of two-dimensional rolls are other kilometer-scale eddies whose energy exchanges with the tolls may be important.
The importance of inflectional instability is indicated by the similarity of roll structure to that predicted by models based on the formation of the rolls as a result of instability in the cross-wind (V component of the Ekman spiral. Rolls observed are generally oriented from 10° to 20° to the left of the wind at inversion base, with maximum roll vertical velocity at 0.33zi(where zi is inversion height) and maximum lateral velocity at 0.07zi Atmospheric roll magnitude compare favorably to those predicted by Brown, but predictions are consistently low with a maximum underestimate of 40%.
Both tower and aircraft measurements indicate substantial heat flux by rolls. It is shown that including positive roll heat flux into Brown's neutral equilibrium energy budget will lead to rolls of larger magnitude.
Abstract
The wind and temperature fields of the Planetary boundary layer (PBL) are investigated during periods in which horizontal roll vortices are present. Measurements from a 444 m tower and from inertially-stabilized aircraft indicate the rolls are maintained primarily by 1) production of energy from the cross-roll component of the mean PBL wind spiral (inflectional instability and 2) buoyancy. Complicating a simple picture of two-dimensional rolls are other kilometer-scale eddies whose energy exchanges with the tolls may be important.
The importance of inflectional instability is indicated by the similarity of roll structure to that predicted by models based on the formation of the rolls as a result of instability in the cross-wind (V component of the Ekman spiral. Rolls observed are generally oriented from 10° to 20° to the left of the wind at inversion base, with maximum roll vertical velocity at 0.33zi(where zi is inversion height) and maximum lateral velocity at 0.07zi Atmospheric roll magnitude compare favorably to those predicted by Brown, but predictions are consistently low with a maximum underestimate of 40%.
Both tower and aircraft measurements indicate substantial heat flux by rolls. It is shown that including positive roll heat flux into Brown's neutral equilibrium energy budget will lead to rolls of larger magnitude.
Abstract
Horizontal roll vortices influence the distribution of turbulence, with turbulence variances and fluxes concentrated in regions of positive roll vertical velocity ωr. This “modulation” of turbulence can be explained simply in terms of the advection of turbulence-generating elements by rolls.
A budget equation is derived for the roll-modulated turbulence energy. Evaluations of various terms in the equation shows that the modulation of turbulence variance is accounted for primarily by a similar modulation in mechanical and buoyancy production near the surface and by vertical transport at higher levels (∼100 m). Energy exchange between rolls and turbulence is relatively unimportant. That is, the rolls modulate, turbulence energy mainly by redistributing turbulence and turbulence-producing elements, rather than by exchanging energy.
Similarly, it is shown that the exchange of energy between rolls and roll-modulated turbulence contributes considerably less to the energy equation of rolls than does the major term, buoyancy.
Abstract
Horizontal roll vortices influence the distribution of turbulence, with turbulence variances and fluxes concentrated in regions of positive roll vertical velocity ωr. This “modulation” of turbulence can be explained simply in terms of the advection of turbulence-generating elements by rolls.
A budget equation is derived for the roll-modulated turbulence energy. Evaluations of various terms in the equation shows that the modulation of turbulence variance is accounted for primarily by a similar modulation in mechanical and buoyancy production near the surface and by vertical transport at higher levels (∼100 m). Energy exchange between rolls and turbulence is relatively unimportant. That is, the rolls modulate, turbulence energy mainly by redistributing turbulence and turbulence-producing elements, rather than by exchanging energy.
Similarly, it is shown that the exchange of energy between rolls and roll-modulated turbulence contributes considerably less to the energy equation of rolls than does the major term, buoyancy.
A survey of women atmospheric scientists has been completed in late 1973 and is briefly summarized in this article. Complete results are on file at the Headquarters of The American Meteorological Society. For the United States, the survey comprised as many as possible of those females either with baccalaureate degrees or higher in atmospheric sciences and those working several years professionally in the field. The names of 247 women who met this definition came to our attention, excluding about 80 more listed as undergraduate majors. The 247 women were subdivided into the following categories: 1) 32 Ph.D.'s; 2) 76 non-student masters' degrees; 3) 84 non-student bachelors' degrees or equivalent; 4) 25 Ph.D. candidates and; 5) 30 masters' candidates. We were able to reach the majority of these women by questionnaire, letter and/or telephone.
The main findings were: 1) the high degree of productive employment in the Atmospheric Sciences or a closely related field (101 of 190 non-students) and 2) the sex-related difficulties noted by the majority (73% of the 150 non-students reporting). All the Ph.D.'s were employed full or part-time in atmospheric science or a related field, and most had received some significant honors. Many of the masters and bachelors had also had outstanding careers, although, in general, traceability and commitment decreased somewhat with the less advanced degrees. The prospects for young women are brighter now than a generation ago. Four young female atmospheric scientists are currently assistant professors in major atmospheric science departments and several are in rising professional situations in the operational weather services. The sex-related difficulties were of two distinct types, namely: 1) discrimination, now decreasing and 2) for married women, problems of co-location of careers with those of husbands and to an important but lesser degree, child care.
The survey exploded the myth that “it does not pay” to provide women with training and opportunity in atmospheric science. Nearly all areas of atmospheric science today have already been infiltrated by females, except that of top management. Related to this, the distributions of males versus females in the field are quite different. With females, 60% are in teaching and/or research, 32% in operations, and 8% in management, while the corresponding figures for males are 28%, 43% and 29% respectively. A surprising result of our survey was the high fraction of foreign born women in the advanced degree categories, decreasing to negligible with the baccalaureates.
A survey of women atmospheric scientists has been completed in late 1973 and is briefly summarized in this article. Complete results are on file at the Headquarters of The American Meteorological Society. For the United States, the survey comprised as many as possible of those females either with baccalaureate degrees or higher in atmospheric sciences and those working several years professionally in the field. The names of 247 women who met this definition came to our attention, excluding about 80 more listed as undergraduate majors. The 247 women were subdivided into the following categories: 1) 32 Ph.D.'s; 2) 76 non-student masters' degrees; 3) 84 non-student bachelors' degrees or equivalent; 4) 25 Ph.D. candidates and; 5) 30 masters' candidates. We were able to reach the majority of these women by questionnaire, letter and/or telephone.
The main findings were: 1) the high degree of productive employment in the Atmospheric Sciences or a closely related field (101 of 190 non-students) and 2) the sex-related difficulties noted by the majority (73% of the 150 non-students reporting). All the Ph.D.'s were employed full or part-time in atmospheric science or a related field, and most had received some significant honors. Many of the masters and bachelors had also had outstanding careers, although, in general, traceability and commitment decreased somewhat with the less advanced degrees. The prospects for young women are brighter now than a generation ago. Four young female atmospheric scientists are currently assistant professors in major atmospheric science departments and several are in rising professional situations in the operational weather services. The sex-related difficulties were of two distinct types, namely: 1) discrimination, now decreasing and 2) for married women, problems of co-location of careers with those of husbands and to an important but lesser degree, child care.
The survey exploded the myth that “it does not pay” to provide women with training and opportunity in atmospheric science. Nearly all areas of atmospheric science today have already been infiltrated by females, except that of top management. Related to this, the distributions of males versus females in the field are quite different. With females, 60% are in teaching and/or research, 32% in operations, and 8% in management, while the corresponding figures for males are 28%, 43% and 29% respectively. A surprising result of our survey was the high fraction of foreign born women in the advanced degree categories, decreasing to negligible with the baccalaureates.
